1. Field of the Invention
The invention relates generally to a highly integrated multimedia processor having tightly coupled functional units and more specifically to sharing a cache between the central processing and graphics units.
2. Description of Related Art
The following background information is provided to aid in the understanding of the application of the present invention and is not meant to be limiting to the specific examples set forth herein. Displaying 3D graphics is typically characterized by a pipelined process having tessellation, geometry and rendering stages. The tessellation stage is responsible for decomposing an object into geometric primitives (e.g. polygons) for simplified processing while the geometry stage is responsible for transforming (e.g. translating, rotating and projecting) the tessellated object. The rendering stage rasterizes the polygons into pixels and applies visual effects such as, but not limited to, texture mapping, MIP mapping, Z buffering, depth cueing, anti-aliasing and fogging.
The entire 3D graphics pipeline can be embodied in software running on a general purpose CPU core (i.e. integer and floating point units), albeit unacceptably slow. To accelerate performance, the stages of the graphics pipeline are typically shared between the CPU and a dedicated hardware graphics controller (a.k.a. graphics accelerator). The floating-point unit of the CPU typically handles the vector and matrix processing of the tessellation and geometry stages while the graphics controller generally handles the pixel processing of the rendering stage.
Reference is now made to FIG. 1 that depicts a first prior art system of handling 3D graphics display in a computer. Vertex information stored on disk drive 100 is read over a local bus (e.g. the PCI bus) under control by chipset 102 into system memory 104. The vertex information is then read from system memory 104 under control of chipset 102 into the L2 cache 108 and L1 cache 105 of CPU 106. The CPU 106 performs geometry/lighting operations on the vertex information before caching the results along with texture coordinates back into the L1 cache 105, the L2 cache 108 and ultimately back to system memory 104. A direct memory access (DMA) is performed to transfer the geometry/lighting results, texture coordinates and texture maps stored in system memory 104 over the PCI bus into local graphics memory 112 of the graphics controller 110 for use in rendering a frame on the display 114. In addition to storing textures for use with the graphics controller 110, local graphics memory 112 also holds the frame buffer, the z-buffer and commands for the graphics controller 110.
A drawback with this approach is inefficient use of memory resources since redundant copies of texture maps are maintained in both system memory 104 and the local graphics memory 112. Another drawback with this approach is the local graphics memory 112 is dedicated to the graphics controller 110, is more expensive than generalized system memory and is not available for general-purpose use by the CPU 106. Yet another drawback with this approach is the attendant bus contention and relatively low bandwidth associated with the shared PCI bus. Efforts have been made to ameliorate these limitations by designating a xe2x80x9cswap areaxe2x80x9d in local graphics memory 112 (sometimes misdescriptively referred to as an off chip L2 cache) so that textures can be prefetched into local graphics memory 112 from system memory 104 before they are needed by the graphics controller 110 and swapped with less recently used textures residing in the texture cache of the graphics controller 110. The local graphics memory swap area merely holds textures local to the graphics card (to avoid bus transfers) and does not truly back the texture cache as would a second level in a multi-level texture cache. This approach leads to the problem, among others, of deciding how to divide the local graphics memory 112 into texture storage and swap area. Still yet another drawback with this approach is the single level texture cache in prior art graphics controllers consume large amounts of die area since the texture cache must be multi-ported and be of sufficient size to avoid performance issues.
Reference is now made to FIG. 2 that depicts an improved but not entirely satisfactory prior art system of handling 3D graphics display in a computer. The processor 120, such as the Pentium II(trademark) processor from Intel corporation of Santa Clara Calif., comprises a CPU 106 coupled to an integrated L2 cache 108 over a so-called xe2x80x9cbacksidexe2x80x9d bus 126 that operates independently from the host or so-called xe2x80x9cfront-sidexe2x80x9d bus 128. The system depicted in FIG. 2 additionally differs from that in FIG. 1 in that the graphics controller 110 is coupled over a dedicated and faster AGP bus 130 through chipset 102 to system memory 104. The dedicated and faster AGP bus 130 permits the graphics controller 110 to directly use texture maps in system memory 104 during the rendering stage rather than first pre-fetching the textures to local graphics memory 112.
Although sourcing texture maps directly out of system memory 104 mitigates local graphics memory constraints, some amount of local graphics memory 112 is still required for screen refresh, Z-buffering and front and back buffering since the AGP bus 130 cannot support such bandwidth requirements. Consequently, the system of FIG. 2 suffers from the same drawbacks as the system of FIG. 1, albeit to a lesser degree. Moreover, there is no way for the graphics controller 110 to directly access the L2 cache 108 that is encapsulated within the processor 120 and connected to the CPU 106 over the backside bus 126.
From the foregoing it can be seen that memory components, bus protocols and die size are the ultimate bottleneck for presenting 3D graphics. Accordingly, there is a need for a highly integrated multimedia processor having tightly coupled central processing and graphical functional units that share a relatively large cache to avoid slow system memory access and the requirement to maintain separate and redundant local graphics memory.
To overcome the limitations of the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a highly integrated multimedia processor employing a shared cache between the central processing and graphics units which may be used in, among other things, a hierarchical texture cache scheme. The graphics unit issues data reads with physical addresses to locations that are cached in the shared cache. If the shared cache misses, a cache fill is performed similar to a cache fill that occurs with a miss from the central processor unit. Regions in the shared cache can also be selectively locked down (thereby disabling eviction or invalidation of data from a selected region) to provide the graphics unit with a local scratchpad area for applications such as line buffering to hold decompressed video for further combination (e.g. filtering) with frame buffer data and composite buffering for blending texture maps in multi-pass rendering. Other 3D applications for the locked down regions may include but are not limited to, bump mapping, Z buffering, W buffering and 2D applications such as blit buffering.
A feature of the present invention is the shared cache provides the graphics unit access to data generated by the central processing unit before the data is written-back or written-through to system memory.
Another feature of the present invention is reduction of the system memory bandwidth required by the central processing and graphics units.
Another feature of the present invention is pushing data transfer bottlenecks needed for 3D graphics display into system memory such that system performance will scale as more advanced memories become available.
These and various other objects, features, and advantages of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a specific example of a multimedia processor employing a shared cache between the central processing and graphics units in accordance with the principles of the present invention.